1. Field of the Invention
The present invention relates to composite materials, and in particular to materials suitable for surgical implantation as replacements for various hard and soft tissue structures.
2. Description of the Related Art
Historically, materials used in endoprosthesis (i.e., the replacement of internal anatomical structures with artificial appliances) have largely been "bioinert". Metallic alloys, such as stainless steel or cobalt chromium, are typically superior in strength to the structures they replace but do not interact chemically or interfacially with surrounding tissue. Although they avoid the many problems arising from tissue is incompatibility, bioinert materials can never become fully integrated within their in vivo environment. As a consequence, the prosthesis frequently detaches from the tissue to which it was originally affixed, resulting in prosthetic loosening. Moreover, modulus mismatching between the appliance and the replaced structure can lead to stress shielding, resulting in poor mechanical compatibility. Bioinert ceramics such as alumina, for example, are stiffer than bone and also exhibit inferior fracture toughness.
An alternative approach is disclosed in U.S. Pat. No. 5,017,627, which sets forth various compositions that, when fabricated and implanted as prosthetic devices, remain anchored to surrounding tissue. The composite materials described in the '627 patent are based on a polyolefinic binder containing a particulate inorganic solid. Disclosed particulate solids include calcium salts such as hydroxyapatite (HA) or fluorapatite, chalk, flyash and silica. Instead of remaining biologically inert, the composite materials instead exhibit "bioactive" behavior, establishing interfacial bonds to compact bone. The ratio of polyolefin to particulate material can be varied to obtain different values of Young's modulus and strain-to-failure and different amounts of interfacial bonding. Importantly, the composite can be made ductile.
While versatile, this type of material exhibits certain limitations. In particular, the range of mechanical properties obtainable according to the '627 patent is relatively limited due to the high HA loading levels necessary to achieve bioactivity. The available values of Young's modulus, for example, tend to be comparable with compact (cortical) bone, but not cancellous bone or soft tissues.
Moreover, soft tissues (such as tendons, ligaments, cartilage and skin) tend to be among the most resistant to adhesion altogether. Even composites containing very high HA concentrations do not stimulate significant interfacial bonding in such tissues. Thus, current materials are both mechanically and chemically unsuited as prostheses for repair of soft-tissue structures.